Benzylacetone is a valuable fragrance and flavor which occurs in the cocoa bean. Its odor is similar to the scent of jasmine and strawberry. Benzylacetone is used in fragrance compositions and aromas (Malasiewicz, Parfxc3xcmerie und Kosmetik, p. 211, 4/88, K. Bauer, A. Garbe, Common Fragrance and Flavor Materials, p. 73, VCH, Weinheim, 1985).
It is known to prepare benzylacetone (1-phenyl-3-butanone) by selective hydrogenation of the double bond of benzylidene acetone. During the hydrogenation, the carbonyl group and the aromatic ring are retained. 
For the hydrogenation with hydrogen, use is made, for example, of nickel catalysts (Chem. Pharm. Bull., 1990, 38(6), 1720), in particular Raney nickel (Ber. Dtsch. Chem. Ges. 56,2174 (1923)).
PL 153267 describes an activated nickel catalyst on aluminum oxide, with which benzylacetone can be prepared in 82% yield from benzylideneacetone at 100-150xc2x0 C.
For the preparation of benzylacetone, noble metal catalysts, such as platinum black, are also known (Ann. Chim. 9, (1), 90, (1914)).
It is also known to prepare benzylacetone using a catalyst of palladium on activated carbon in a solution of acetic acid/ammonium formate with a yield of 78% (Synth. Commun. (1998), 28(22), 4193).
It is also known to prepare benzylacetone using a catalyst of palladium acetate in dimethylformamide in the presence of potassium formate with a yield of 97% (Synlett. 1991,1, 27).
It is also known to prepare benzylacetone using a catalyst of palladium on silicon dioxide/aluminum phosphate in methanolic solution with a selectivity of 100% (React. Kinet. Catal. Lett, 25, 45-50 (1984)).
It is also known to prepare benzylacetone using a catalyst combination of palladium or platinum with the rare earths, such as lanthanum, praseodymium (DE-A 2 615 308).
In the case of industrial-scale production, the known processes for the preparation of benzylacetone from benzylidene acetone exhibit disadvantages by virtue of the toxicity of the catalyst, by virtue of the uneconomical nature because of the dilution with solvents, by virtue of yields which are too low or by virtue of catalysts which have to be prepared in a complex manner.
There is still the need to find a process which gives benzylacetone in an ecologically acceptable and economical manner.
We have found a process for the preparation of benzylacetone by hydrogenation of benzylideneacetone, which is characterized in that the hydrogenation is carried out in the presence of a palladium catalyst on activated carbon and/or a palladium catalyst on aluminum oxide.
The process according to the present invention is preferably carried out without solvents.
Using the process according to the present invention, it is possible to prepare benzylacetone in high yields in an economic manner. The handling of palladium as catalyst is particularly advantageous since the catalyst does not represent a hazardous substance.
For the process according to the present invention, the palladium catalyst can be used in the dry or moist (residual moisture of water) state.
For the process according to the present invention, the proportion of palladium on the support material of activated carbon and/or aluminum oxide can be 1 to 50% by weight, preferably 5 to 10%, based on the dry catalyst.
For the process according to the present invention, the weight ratio of the catalyst used to benzylideneacetone is preferably 0.00001 to 0.01, more preferably 0.0001 to 0.001:1.
The reaction temperature for the process according to the present invention is preferably 20 to 100xc2x0 C., more preferably 50-80xc2x0 C.
The hydrogen pressure is preferably 1 to 20 bars, more preferably 1 to 5 bars.
The reaction time is preferably 1 to 20 hours, more preferably 6 to 15 hours.
The process according to the present invention can be carried out as follows:
A reaction vessel with gas-dispersion stirrer is charged with benzylideneacetone and e.g. palladium on activated carbon 5% by weight. Hydrogenation is carried out at a hydrogen pressure of e.g. 1 bar. The crude product is filtered and distilled.
It is surprising that the hydrogenation can be carried out in high yield and selectively in the presence of a palladium catalyst on activated carbon and/or a palladium catalyst on aluminum oxide.